1. Field to Which the Invention Belongs
The present invention relates to an organic light emitting element having an anode, a cathode, and a layer containing an organic compound that emits light upon application of electric field (hereinafter referred to as “organic compound layer”), and to a light emitting device using the organic light emitting element. In general, light emission obtained from organic compounds by application of electric field is divided into light emission upon return to the base state from singlet excited state (fluorescent light) and light emission upon return to the base state from triplet excited state (phosphorescent light). The present invention relates particularly to an organic light emitting element using an organic compound that can emit phosphorescent light. Note that, in this specification, a light emitting device refers to an image display device that uses an organic light emitting element as its light emitting element. Also, the following modules are all included in the definition of the light emitting device: a module obtained by attaching to an organic light emitting element a connector such as an anisotropic conductive film (FPC: flexible printed circuit), a TAB (tape automated bonding) tape, or a TCP (tape carrier package); a module in which a printed wiring board is provided at an end of the TAB tape or TCP; and a module in which an IC (integrated circuit) is directly mounted to an organic light emitting element by the COG (chip on glass) method.
2. Prior Art
Organic light emitting elements are elements that emit light upon application of electric field. It is said that organic light emitting elements emit light through the following mechanism: a voltage is applied between electrodes that sandwich an organic compound layer, electrons injected from the cathode and holes injected from the anode are re-combined in the organic compound layer to form excited molecules (hereinafter referred to as “molecular excitons”), and the molecular excitons return to the base state while releasing energy to cause the organic light emitting element to emit light.
In an organic light emitting element as above, its organic compound layer is usually a thin film of sub-micron level. In addition, the organic light emitting element does not need back light used in conventional liquid crystal displays because it is such a self-luminous element that the organic compound layer itself emits light. The organic light emitting element is therefore useful in manufacturing a very thin and light-weight device, which is a great advantage.
When the organic compound layer is about 100 to 200 nm in thickness, for example, recombination takes place within several tens nanoseconds or so since carriers are injected based on the mobility of carriers in the organic compound layer and, counting the process from carrier recombination to light emission, the organic light emitting element is readied for light emission in order of equal to or less than microseconds. Accordingly, fast response is also one of the features of the organic light emitting element.
Furthermore, since the organic light emitting element is of carrier injection type, it can be driven with direct-current voltage and hardly produces noises. Regarding drive voltage, a report says that a sufficient luminance of 100 cd/m2 is obtained at 5.5 V by using a very thin film with a uniform thickness of about 100 nm for the organic compound layer, choosing an electrode material capable of lowering a carrier injection barrier against the organic compound layer, and introducing the hetero structure (two-layer structure). (Reference 1: C. W. Tang and S. A. VanSlyke, “Organic electroluminescent diodes”, Applied Physics Letters, Vol. 51, No. 12, 913–915 (1987))
Organic light emitting elements are drawing attention as the next-generation flat panel display elements for their characteristics including being thin and light-weight, fast response, and direct-current low-voltage driving. Also, being self-luminous and having wide viewing angle give the organic light emitting elements better visibility. Therefore the organic light emitting elements are considered as effective elements for display screens of portable equipment.
By the way, as mentioned above, light emission in organic light emitting elements is a phenomenon that is generated at the time when molecular excitons return to the base state. However, the types of molecular excitons of organic compounds may include a singlet excited state (S*) and a triplet excited state (T*). The statistical ratio of generation of the two states in organic light emitting elements is considered as S*:T*=1:3 (Reference 2: Junji Kido, “Monthly Display Special Issue, Organic EL Display: From the Basics to the Latest News”, Techno Times Co., Ltd., p. 28–29).
However, in case of general organic compounds, light emission (phosphorescent light) from the triplet excited state (T*) at room temperature is not observed, and it is usually observed only light emission (fluorescent light) from the singlet excited state (S*). This is because the base state of organic compounds is normally the singlet base state (So), and therefore the transition from T* to So is a highly spin forbidden transition, whereas the transition from S* to So is a spin allowed transition.
Accordingly, only singlet excited state (S*) usually contributes to light emission and this applies to organic light emitting elements, too. Thus, the theoretical limit of the internal quantum efficiency (the ratio of photons generated to the amount of carriers injected) in organic light emitting elements, which is calculated based on S*:T*=1:3, is therefore 25%.
Not all of light generated reach outside. A part of light generated cannot be taken out due to the refractive index innate to materials constituting the organic light emitting element (organic compound layer materials and electrode materials) and to the material of the substrate. The ratio of light that is taken out to all of the light generated is called the efficiency of taking light out. The efficiency of taking light out is said to be around 20% in organic light emitting elements that have glass substrates.
From the above reasons, it has been said that the theoretical limit of the final ratio of photons that can be taken out to the number of carriers injected (hereinafter referred to as “external quantum efficiency”) is 25%×20%=5% even if all of the injected carriers form molecular excitons. In short, only 5% of re-combined carriers are taken out as light even when all of the carriers are re-combined.
In recent years, however, organic light emitting elements that can convert energy released when returning to the base state from triplet excited state (T*) (hereinafter referred to as “triplet excitation state energy) into light emission have been published one after another and their high light emission efficiency is attracting attention. (Reference 3: D. F. O'Brien, M. A. Baldo. M. E. Thompson and S. R. Forrest, “Improved energy transfer in electrophosphorescent devices”, Applied Physics Letters, Vol. 74, No. 3, 442–444 (1999)) (Reference 4: Tetsuo TSUTSUI, Moon-Jae YANG, Masayuki YAHIRO, Kenji NAKAMURA, Teruichi WATANABE, Taishi TSUJI, Yoshinori FUKUDA, Takeo WAKIMOTO and Satoshi MIYAGUCHI. “High Quantum Efficiency in Organic Light-Emitting Devices with Iridium-Complex as a Triplet Emissive Center”, Japanese Journal of Applied Physics. Vol. 38, pp. L1502–L1504 (1999))
In Reference 3, a metal complex having platinum as its central metal (hereinafter referred to as “platinum complex”) is used and Reference 4 employs an organic metal complex having iridium as its central metal (hereinafter referred to as “iridium complex”). Both complexes are characterized in that a third transition series element is introduced as the central metal. Some of them easily exceed the aforementioned theoretical limit value of the external quantum efficiency of 5%.
If layers formed of an iridium complex and layers formed of DCM2 that is a known fluorescent pigment are alternately layered, the triplet excitation energy generated by the iridium complex can be transferred to DCM2 to contribute to light emission of DCM2. (Reference 5: M. A. Baldo, M. E. Thompson and S. R. Forrest, “High-efficiency fluorescent organic light-emitting devices using a phosphorescent sensitizer”, Nature (London), Vol. 403, 750–753 (2000)). DCM2 emits light from singlet excited state (fluorescent light). However, the triplet excitation energy generated by the iridium complex with high efficiency is utilized for the singlet excitation energy of other molecule, namely, DCM2, thereby improving the efficiency.
As shown in Reference 3 to 5, organic light emitting elements using organic compounds that can convert the triplet excitation energy to light emission (hereinafter referred to as “triplet light emission materials”) can achieve higher external quantum efficiency than conventional ones. As the external quantum efficiency is raised, the luminance of emitted light is improved. Accordingly, organic light emitting elements using triplet light emission materials will take up a great portion in future development of light emitting elements as measures for achieving high luminance light emission and high efficiency light emission.
However, platinum and iridium are both so-called precious metals, and therefore a platinum complex and iridium complex using those are also expensive. This is expected to hinder reduction of cost in future. In addition, platinum and iridium are rare metals and there will be difficulties in supplying them in mass production.
The iridium complex emits light of green color, namely, light of an intermediate wavelength in the visible light range. The platinum complex, when used as a dopant, causes the element to emit light in red with relatively good color purity. However, if the concentration of the platinum complex is low, its host material also emits light to degrade the color purity and, if the concentration is high, the light emission efficiency is lowered because of concentration quenching. In short, no one has succeeded in obtaining red light and blue light of high color purity at high light emission efficiency from organic light emitting elements that can convert the triplet excitation energy into light emission.
Furthermore, the iridium complex does not have high productivity because it is an organic metal complex in which the central metal is directly σ-coupled to a benzene ring serving as a ligand, and the synthesis takes time and the yield is poor. A Werner complex, such as tris(8-quinolinolate) aluminum (hereinafter referred to as “Alq3”) that is often used in organic light emitting elements, is generally considered as more effective from the view point of productivity.
To manufacture full-color flat panel displays using elements that emit red light, green light, and blue light in future, it is necessary to mass-produce materials which have as high external quantum efficiency as the platinum complex and iridium complex and which emit light of excellent color purity from less expensive row materials.
Due to the circumstances described above, it is inevitable to develop triplet light emission materials, other than the existing platinum complex and iridium complex.